Compassion is something that is absolutely central to Judaism and the way that one should interrelate with those around him. However, compassion and mercy are two different traits and even though one must try to be compassionate at all times, that does not mean that one should be merciful in a situation that does not allow for such a reaction. For example, the judges who sat on the Sanhedrin were the most compassionate of people but that did not mean that they did not rule according to the law even when it meant not being merciful. In fact, King Saul was censured by God for being "more" merciful than God and leaving Agag the king of Amalek alive.
